Rare Disease Database

General Myoclonus

NORD gratefully acknowledges John N. Caviness, MD, Professor of Neurology, Mayo Clinic College of Medicine, for assistance in the preparation of this report.

Synonyms of General Myoclonus

No synonyms found

Subdivisions of General Myoclonus

action myoclonus

arrhythmic myoclonus

cortical myoclonus

cortical-subcortical myoclonus

dyssynergia cerebellaris myoclonica

familial arrhythmic myoclonus

hereditary essential myoclonus

infantile myoclonic encephalopathy and polymyoclonia

intention myoclonus

Lance-Adams syndrome

myoclonic epilepsy

nocturnal myoclonus

opsoclonus

palatal myoclonus

paramyoclonus multiple

pathological myoclonus

peripheral myoclonus

postanoxic intention myoclonus

postencephalitic intention myoclonus

progressive myoclonic epilepsy

respiratory myoclonus

rhythmical myoclonus

segmental myoclonus

stimulus-sensitive myoclonus

subcortical/non-segmental myoclonus

General Discussion

Myoclonus is the term used to describe the sudden, involuntary jerking of a muscle or group of muscles caused by muscle contractions (positive myoclonus) or muscle relaxation (negative myoclonus). The twitching or jerking of muscles cannot be controlled by the person experiencing it. Myoclonic jerks may occur infrequently or many times a minute. They sometimes occur in response to an external event or when a person attempts to make a movement. By itself, myoclonus may be seen as a symptom rather than a disease. To some degree, it may occur occasionally to otherwise healthy people. (For instance, hiccups may be considered a type of myoclonus.) In severe cases, it can interfere with movement control and balance, and limit various everyday activities such as eating or talking.

Signs & Symptoms

There are different forms of myoclonus and they are classified in different ways. One way of classifying the different forms is by their cause. Some of the types of myoclonus are:

Physiologic myoclonus
This occurs in neurologically normal people. The occurrence of myoclonus during sleep and sleep transitions is the most common example.

Essential myoclonus
In this type, the myoclonic jerks or twitches are usually the most prominent or only clinical finding. This type of myoclonus usually progresses slowly or not at all. There are hereditary (autosomal dominant) and non-inherited, random (sporadic) forms.

Progressive myoclonus epilepsy (PME)
This is a group of diseases characterized by myoclonus, epilepsy, and other symptoms such as trouble walking or speaking. These disorders tend to get worse over time (progressive).

Sleep myoclonus
This typically occurs just at the moment of dropping off to sleep. In some cases, the affected individual does not find it particularly troublesome. In other cases, it may interfere with the sleep process. Myoclonus may be a symptom in certain sleep disorders such as restless legs syndrome.

Symptomatic (secondary) myoclonus
This is the most common category and usually is found in the setting of an identifiable underlying disorder. Myoclonus may not be the most prominent clinical symptom. Common co-existing problems include ataxia, dementia, and Parkinsonism. Myoclonus may also be a symptom associated with infections, non-neurologic medical illnesses, toxic-metabolic states, and storage diseases.

Causes

Myoclonus is caused by an abrupt and brief discharge of motor neurons to affected muscles. In most cases, this results from a disturbance in the central nervous system, although it is believed that in rare cases may be caused by an injury to the nerves outside the central nervous system (peripheral nerves). Several different locations within the brain are thought to be involved in myoclonus. As a result, various types of testing is usually required to define the cause of myoclonus.

The locations that cause myoclonus are reflected in the physiological classification of myoclonus:

Chemicals that carry messages from one nerve cell to another (neurotransmitters) may play a role. In some cases, myoclonus may be present because of an imbalance in these chemicals. However, the specific causes are not well understood at this time.

Affected Populations

Myoclonus affects males and females in equal numbers. Some forms of myoclonus are common and some forms are rare. In general, the incidence of myoclonus is 1.3 cases per 100,000 person-years, and the prevalence is 8.6 cases per 100,000 populations.

Related Disorders

Symptoms of the following disorders can be similar to those of myoclonus. Comparisons may be useful for a differential diagnosis:

Tourette syndrome is a neurologic movement disorder that is characterized by repetitive motor and vocal tics. The first symptoms are usually rapid eye blinking or facial grimaces. Symptoms may also include facial tics and involuntary movements of the extremities, shoulders, and the voluntary muscles. Inarticulate sounds or sometimes inappropriate words may occur. Tourette syndrome is not a progressive or degenerative disorder; rather, symptoms tend to be variable and follow a chronic waxing and waning course. Symptoms usually begin before the age of 16 years. (For more information on this disorder, choose “Tourette” as your search term in the Rare Disease Database.)

Jumping Frenchmen of Maine is a very rare disorder characterized by an extreme startle response. The symptoms occur as a response to sudden, unexpected noise or movement. The extreme startle reaction includes jumping, raising the arms, hitting, yelling, unintelligible speech, and/or imitation or repetition of another person’s body movements (echopraxia). The intensity of the response increases with fatigue and stress. (For more information on this disorder, choose “Jumping Frenchmen of Maine” as your search term in the Rare Disease Database.)

Huntington’s disease (Huntington’s chorea) is a rare inherited, progressively degenerative neurological disorder characterized by involuntary muscle movements and dementia. Initially there are personality changes and uncontrolled rapid jerky muscle movements. In time, speech and memory become impaired and involuntary muscle movements become more frequent and severe. As Huntington’s disease progresses, there is a further loss of cognitive abilities and dementia. The symptoms of this disorder usually begin during adulthood, generally after the age of forty. (For more information on this disorder choose, “Huntington’s” as your search term in the Rare Disease Database.)

Torsion dystonia is a rare inherited neurological disorder characterized by involuntary contortions of the muscles in the neck, torso, arms, and legs. Occasionally only one or a few muscles are involved. People with torsion dystonia typically have an awkward, sideways gait. Other symptoms may include foot drag, cramps on the hands and feet, difficulty in grasping objects, and unclear speech. The involuntary movements of dystonia are slow writhing movements. (For more information on this disorder, choose “Torsion Dystonia” as your search term in the Rare Disease Database.)

Benign essential tremor is a rare neurological disorder characterized by a rhythmical tremor that may be pronounced. This disorder typically affects the upper extremities. The tremors may be aggravated by stress, anxiety, fatigue, and/or cold temperatures. Relief from the tremors may be achieved by rest and sedation. The symptoms of benign essential tremor generally stabilize after a period of progression. (For more information on this disorder, choose “Benign Essential Tremor” as your search term in the Rare Disease Database.)

Standard Therapies

If the cause for the underlying disorder cannot be cured, then the standard treatment for myoclonus is medications that may help reduce symptoms. The first line of therapy depends on where the myoclonus originates within the nervous system (i.e. physiological classification).For cortical myoclonus (most common source), this would include levetiracetam and/or valproic acid, a type of tranquilizer, and other drugs known as benzodiazepine derivatives which include clonazepam. The beneficial effects of certain drugs, including clonazepam, may diminish over time.

Many of the drugs used for myoclonus, such as barbiturates, phenytoin, and primidone, are also used to treat epilepsy. Certain of these drugs may have side effects such as sleepiness, unsteady gait (ataxia), or lethargy, and patients and their families should be aware of these beforehand.

Newer therapies have been suggested for very specific types of myoclonus. Deep brain stimulation may be considered for the inherited Myoclonus-Dystonia Syndrome, a type of Subcortical/Nonsegmental myoclonus. Botulinum toxin has been tried for palatal myoclonus, a type of segmental myoclonus. Extensive physician consultation is needed before these therapies should be performed.

Genetic counseling will be of benefit for patients with the inherited forms of myoclonus and their families.

Investigational Therapies

Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. government funding, and some supported by private industry, are posted on this government web site.

For information about clinical trials being conducted at the National Institutes of Health (NIH) in Bethesda, MD, contact the NIH Patient Recruitment Office:

Some studies have shown that doses of 5-hydroxytryptophan (5-HTP), a building block of serotonin, leads to improvement in patients with some types of myoclonus. However, other studies indicate that this therapy is not effective in all people with myoclonus and may even cause the jerks and twitches to worsen in some affected individuals.

Because myoclonus is complex in nature it may require a combination of drugs for effective treatment. Some drugs currently being studied in different combinations include levetiracetam, clonazepam, sodium valproate, and primidone. In some people, treatment may also include hormonal therapy.

Years Published

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